1. Field
The present disclosure relates to improvements in data transmission and reception using networked receivers having spatial diversity.
2. Description of Related Art
Reliable and accurate transmission of information is paramount in modern society. Not only is transmission of information from terrestrial-based sources widespread, but transmission of information from aerial-based sources is also widespread, becoming more so with time. As an example, aerial-based sources such as Unmanned Aerial Vehicles (UAVs) are gaining broad acceptance in the military as a means of acquiring intelligence information from remote locations where the risk of sending manned vehicles is great.
Often, information transmitted from aerial-based sources takes the form of imagery. For example, one of the most common uses of UAVs is to obtain imagery information, either from cameras operating in the visible light or infra-red ranges, or from small aperture radars or other imagery systems. The imagery information may take the form of either motion-based imagery (video) or still images.
In many applications, there is a strong desire to provide the ability for ground-based analysts to view transmitted imagery in real time. Thus, many UAV video systems digitize the imagery, compress it using a variety of industry-standard or non-standard methods, and transmit resulting data to the ground via a radio frequency (RF) downlink. On the ground, the data may be decompressed and returned to a format that is suitable for viewing on a video monitor.
While many types of information can be reliably and accurately transmitted, transmission of imagery from aerial-based sources is often particularly vulnerable to errors. One common problem in such systems stems from the fact that the RF downlink is typically far less reliable than transmission media commonly used for the transport of digital video from terrestrial-based sources in the consumer marketplace (e.g., cable, storage devices such as CDs, DVDs, etc.). Consequently, the bit stream received from the RF downlink is often subject to errors (i.e., bit errors).
In order to efficiently transmit imagery information, data forming the image is typically compressed before transmission and subsequently decompressed at the receiver. As a result of video compression, a small number of bit errors present in the data can cause significant disruption to the video displayed. For example, given a bit error rate of one in a hundred million, video is disrupted on average once every half minute. At a bit error rate of one in 10 million, video is disrupted on average once every 3 seconds. At a bit error rate of one in 1 million, video is subject to nearly constant disruption. Minimization of bit errors is thus important in the transmission of imagery information, particularly when the information is subjected to video compression.
Errors in the bit stream are typically caused by several mechanisms. These include errors arising from the following: front end noise in the receiver, multipath reflections, interference, and shadowing. Each mechanism alone can introduce substantial errors in the bit stream. Often, however, multiple errors are introduced in the bit stream by simultaneous operation of one or more mechanisms.
With respect to errors introduced by front end noise present in the receiver, it should be noted that the same amount of noise is always present at the receiver. When the received signal is weak (typically because it has propagated for a long distance), however, the front end noise may be sufficiently large to degrade the signal. This phenomenon affects the bit stream in much the same way that reception of a signal from a distant commercial broadcast radio station is negatively affected by noise. Receiver “sensitivity” is a measure of receiver quality in this regard. Receiver sensitivity comprises a minimum signal level sufficient to prevent the receiver front end noise from causing an unacceptable number of bit errors.
With respect to multipath reflections, it should be noted that the received signal often arrives at a receiver antenna through not only a direct path via the air, but also through a secondary path. The secondary path typically arises due to reflection of the signal from the ground (or other object(s)) present in the forefront of the receiver antenna. As a result, a transmitted signal often arises at the receiver in the form of two signals (a first signal being direct, and a second signal being a reflection). Because the two signals travel paths having different lengths, they can arrive at the antenna with different phasing. In some cases the phases are the same, in which case the reflection signal serves to boost the strength of the direct signal. In other cases the phases are misaligned. When the phases are misaligned the sum of the two signals is attenuated. When the misaligned phases cause the composite sum of the signal to be attenuated, the signal is said to be “in a null.” If the signal strength in the null is too close to the sensitivity of the receiver, bursts of errors often occur when the signal enters the null. This can occur in a time varying manner, as when, for example, an aircraft signal source moves from one location to another.
For purposes of the present disclosure, interference comprises unwanted signals received or introduced by other sources. These other sources may be generated intentionally by a hostile party trying to disrupt communication (e.g., by a jammer) or they may be generated unintentionally by a friendly origin that lacks adequate control over the frequency spectrum that it occupies. One example of the latter source is pulsed radar. If the interference signal at the receiver antenna is close in magnitude to that of the desired signal, errors can be introduced into the bit stream.
In general, when transmitting, the transmit antenna is usually attached to the source (e.g., an aircraft) in some fashion. When transmitting from an aerial-based source such as an aircraft, it is virtually impossible to avoid mounting the antenna in a position where at some particular attitude of the aircraft, the signal path from the aircraft antenna to the ground antenna does not pass through the aircraft structure. In this case, the aircraft structure may significantly attenuate the signal received by the ground-based receiver, which can cause the signal strength received to fall below the receiver sensitivity level. This error-causing mechanism is referred to as “shadowing”.
There are many techniques that can be used to combat errors in such environments. These include the following: increasing transmitter power level, reducing the bit rate, use of coding such as forward error correcting (FEC) coding, use of directional antennas with antenna pointing, and use of diversity techniques. More than one technique for combating errors can be used for increased performance.
Increasing transmitter power levels allows the transmitted signal to experience more attenuation before it is negatively impacted by the receiver front end noise or interference. In this manner, the transmitter power level is increased to a level such that, when attenuated, the signal strength at the receiver remains sufficiently large to avoid being significantly impacted by noise or interference.
Reducing transmission bit rate also works to combat errors in a manner similar to increasing transmitter power levels. The performance of any communication link is a function of how much energy is available at the receiver per bit of data (i.e., energy per bit). If the bit rate of a signal is reduced, and everything else about the communication link remains unchanged, the energy per bit increases in direct proportion to the reduction in bit rate. This increase provides enhanced margin between the power of the desired signal and that of the receiver front end noise.
Coding, such as Forward Error Correction (FEC) coding, generally involves a digital signal processing technique that allows correction of a large percentage of the bit errors in a corrupted bit stream. However, the benefits come at the cost of requiring transmission of some additional overhead data bits. The overhead data bits are generated as a function of the information being transmitted, and they are added to the original data bits with the intent that the code formed from the combination of original and overhead data bits does not allow every possible combination of bits. Because not all bit patterns are possible in the code, a decoder can correct errors in a manner that is analogous to the way misspellings in text are detectable because not all combinations of letters are part of the “code” formed by language itself. FEC is an important tool, but as the channel quality decreases, more and more redundancy must be added to achieve a given level of performance. The required bandwidth for addition of such redundancy is often not available.
According to further variations, FEC coding can be used in conjunction with interleaving. Interleaving is a method of dispersing the effects of errors grouped together so that the FEC (which typically works best when errors are uniformly distributed) can perform adequately.
As noted, using directional antennas with antenna pointing is another known technique for combating interference. If a directional antenna is pointed at a UAV, for example, it provides substantial gain to the desired signal and also provides loss for any interference source that is not transmitting in the same direction as the UAV.
The term “diversity” refers to a family of techniques in which a signal is received in more than one manner, with the assumption that the errors that occur in each receiver are, to some extent, independent. There are a number of known diversity techniques. Most of the diversity techniques involve having more than one receiver operate on the same signal, and having some manner of selecting the result of the receiver that is doing the best job of receiving the signal. Some diversity techniques combine signals from all of the receivers before determining a best estimate of the transmitted data. Typical diversity techniques include those based on the following: frequency diversity, time diversity, and spatial diversity.
In accordance with frequency diversity techniques, the same signal is transmitted and received on two different frequencies. Because the multipath reflection and interference characteristics differ at different frequencies, at any given time, the signal received at one frequency may be acceptable, while the signal received at the other frequency is unacceptable.
In accordance with time diversity techniques, the same signal is transmitted at different times. This technique is beneficial when, for example, interference occurs at a first instant in time but not at another instant in time.
In accordance with spatial diversity techniques, the same signal is received at different spatial locations (such that different paths of signal travel are present). This technique is beneficial when, for example, propagation loss and interference are different at each spatial location where the signal is transmitted or received.
Many diversity systems exist in the art. One of the most widely used examples of diversity receivers is found in base stations for cellular telephony. Cellular telephony base stations typically have multiple antennas and receivers from which the best signal is selected from among the available receivers. The receivers are typically co-located, with the receiving antennas being spaced apart only by a sufficient distance (e.g., less than a few meters) such that multipath reflections that each receiving antenna experiences are unrelated to those experienced by other receiving antennas. The relatively small spacing of receiving antennas in such systems is typically not sufficiently large to combat errors introduced by shadowing and interference.
Another known diversity system is that developed in the late 1980s for use on U.S. Air Force and Navy training ranges (the Range Applications Joint Program Office (RAJPO) Data Link System (DLS)). According to the RAJPO DLS, multiple ground stations positioned at significantly different sites throughout training ranges could be used to collect transmitted downlink information. This system, however, relied upon dedicated line-of-sight (LOS) microwave links to facilitate the transmission of data from the ground stations to a single receiver at a central processing site, where the data was then combined using custom hardware. The links from the ground stations were fully synchronous and timing was thus tightly controlled among the different ground stations. While this system had its benefits, it also had limitations—one of which involved the vulnerable LOS links between the ground stations and the central processing site.
Many systems employing spatial diversity are not capable of differentiating between individual signals and providing the best individual output at a given instant in time based on the signals so received. For example, many known techniques employing spatial diversity rely on equal gain combining whereby all signals are combined together regardless of the strength of any particular individual signal. As another example, many known techniques employing spatial diversity rely on optimal combining whereby the signals are combined proportionally based on their individual strengths.
Data transmission systems may use several and possibly all of the above-described error reduction techniques, or others, at once. Although there are many known ways to effectively implement each of these techniques, there is a continual need for improvements in accuracy and reliability of transmission, particularly when transmitting complex data such as imagery.
The data transmission method and apparatus using networked receivers having spatial diversity provides an improved data transmission system, especially when transmitting complex imagery information.